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What are Mexican jumping beans?

By Chelsie Vandaveer

January 19, 2005

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The jumping bean (Sebastiania pavoniana (Müller Argoviensis) Müller Argoviensis) is a member of the Euphorbiaceae, the spurge family. It is a desert plant found in the arroyos and on slopes in the Sonoran and Chihuahuan deserts southward to the tropical dry forests of Central America. Typical of the family, the jumping bean contains latex, yellow and toxic. The latex is bitter and tacky; herbivores learn to leave the plant alone.

Saguaros Cacti Rise from the Sonoran Desert, Arizona-Mexico Border
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The jumping bean is a small shrubby tree with alternate elliptic leaves with serrated edges. Brian Enquist and Jon Sullivan describe the leaves as "uniformly a creamy green [with] a 'plastic' feel". The leaves turn brilliant red in late autumn and drop in spring.

After the monsoonal winds of summer bring rain to the deserts, the jumping bean blooms. The flowers are tiny, not much bigger than a pinhead. If pollinated, the flower will produce a pod with three seeds each in its own wedge-shaped carpel. When ripe, the carpels separate and drop from the shrub.

In the late spring months that precede the summer rains, the deserts and dry forests suffer from staggering drought. Temperatures climb and desert plant life goes into waiting mode—cacti shrink, shrubs and trees drop their leaves, and seeds lie dormant on the ground. But around the jumping bean shrubs, there is an odd noise, a sound like pattering raindrops on dry leaves.

Great Chihuahua Desert with Chisos Mountains and Mt Amory at Big Bend National Park Photographic Print Ralph Crane Buy Photographic Print at AllPosters.com

The previous year during flowering, a gray moth (Cydia saltitans (Westwood)) laid an egg or two on the ovary of the jumping bean flowers. The caterpillar ate its way into one of the carpels and consumed the seed. It chewed a hatch and anchored itself inside the carpel with silk. By the time the carpel fell to the earth, the caterpillar was ready to pupate. Anchored, the caterpillar tossed itself inside the carpel causing saltation, the jumping or rolling action of the 'bean'.

It is not understood why the caterpillar throws itself and hence the 'bean' around. It is suggested that perhaps the motion moves the carpel into a shadier location or someplace safer for the caterpillar to pupate. After the rains arrive and the Sebastiania comes into flower, a small moth emerges from the hatch and seeks a mate.

Beans that Jump, a commercial website, has a great page that shows the life cycle of Cydia saltitans (=Laspeyresia saltitans) showing the pupal cases, carpels, and an adult moth. To view the page, click on the link:

http://www.beansthatjump.com/life_cycle.html

The Arizona-Sonora Desert Museum is located in Tucson and a great place to visit on your way to explore the Sonoran. To learn more about the museum and the amazing Sonora Desert, click on the link:

http://www.desertmuseum.org/

(Compiled from: "Sebastiania pavoniana", W3TROPICOS, Jim Solomon, Missouri Botanical Garden; Deserts, James A. MacMahon, Audubon Society Nature Guides, Alfred A. Knopf, New York, 1985; and "Vegetative key and descriptions of tree species of tropical dry forests of upland Sector Santa Rosa, Area de Conservación Guanacaste, Costa Rica", Brian J. Enquist and Jon J. Sullivan, posted on the internet by the University of Arizona Ecology and Evolutionary Biology; and Beans that Jump.)

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